The gangs are using classic shoplifting techniques to distract gallery owners in a crime wave which has netted bronzes which are together valued at more than pounds 500,000.

Insurance companies are alarmed. Mark Dalrymple, chairman of the Council for the Prevention of Art Theft, said: "They are nothing more than shoplifters who have realised it's much easier to go into a gallery and take something worth pounds 25,000 than lift a load of gear from Marks & Spencer which is only worth pounds 100."

The chief targets are galleries in the West End of London, but incidents have also been reported in Bath and Harrogate.

One gang is described in a security bulletin issued to art galleries as like "extras in the television soap opera EastEnders in the way they dress and act". Some carry long coats over their arms for camouflage and others distract gallery staff.

The security bulletin warns: "Some dealers' staff are too casual about unlikely customers entering galleries asking about the prices of bronzes or other works. These are individuals who are clearly not in the market to buy. They may well be in the gallery to steal."

In May two men were arrested when a Henry Moore bronze was found in the back of a taxi which was stopped for a routine inquiry at a security checkpoint in the City of London. The statue, valued at pounds 50,000, had been reported stolen a month earlier from the Waddington Galleries.

Charles Hill, risk manager at art insurers Nordstern and the former head of the arts and antiques squad at Scotland Yard, suggested that bronze statues should be wired down or fitted with security alarms. He also advised galleries to fit closed circuit television cameras and to ask customers to hand in their coats and bags.

Galleries fear new security measures will hinder them interesting the wider public in art. Neil Smith, secretary of the Society of London Art Dealers, said: "We are trying to educate people and trying to encourage people into galleries but how on earth do you combine that with the measures that are needed?"